The miniaturization of mobile devices such as a mobile phone and PDA (Personal Digital Assistant) has been progressing and the performance thereof has been improved more and more in recent years, and the BGA (Ball Grid Array) and the CSP (Chip Scale Package) have been used in many cases as the mounting technology which can accommodate the miniaturization and the performance improvement. However, because of the demand for the further miniaturization and thickness reduction, a thin-type package such as the LGA (Land Grid Array) in which even the solder bump height is reduced has been increasingly adopted. However, the thin-type package has the problems as follows.
First, in the case where the package size is increased, the influence of warp of the package itself or the substrate becomes large and the connection failure due to the warp occurs at the time of reflow. Also, since the amount of solder used for the connection is reduced in the thin-type package, the package/substrate distance after the connection is reduced, and the growth rate of the crack formed at the solder connecting portion due to the difference in the linear expansion coefficient between the package and the substrate is increased. Consequently, the thermal fatigue life is shortened.
Furthermore, the lead-free solder has been widely used in recent years. The Sn—Ag—Cu series solder which is highly reliable and is the most dominant lead-free solder has the melting point of 217° C., which is more than 30° C. higher than the melting point of conventionally used Sn37Pb solder of 183° C. Therefore, it is necessary to increase the reflow temperature, and hence, it is difficult to mount the low heat resistant parts. For the solution of the problem, low melting point Pb free solders such as Sn—Zn series solder and Sn—In series solder has come into use. However, it is known that the strength of the connection interface is reduced when the Sn—Zn series solder of the low melting point solders is used for the connection to the pad having a metallization (e.g. metallized layer) containing Au.
The result of the study as a premise of the present invention by the inventors thereof will be described here. FIG. 12 shows the result of evaluation of the strength of the connection interface by means of the shear test to the samples in which an Sn8Zn3Bi solder ball is connected to a pad having metallization of Au/Pd/an electroless Ni—P plating while changing the reflow temperature and the reflow time. In this shear test, as shown in FIG. 12A, the sample is inclined at 25° so as to sufficiently apply the load to the connection interface, and the sample is shorn at the high velocity of 4 mm/s (mm/s: Millimeter per Second). In this evaluation by the shear test, as is understood from the results of FIG. 12B (reflow temperature: 250° C.), FIG. 12C (reflow temperature: 230° C.) and FIG. 12D (reflow temperature: 210° C.), the shearing strength is reduced under the reflow conditions of high reflow temperature and long reflow time.
Also, the solder/metallization interfacial breaking occurs under all of the conditions in this evaluation. FIG. 13 shows the observation results of the cross section of the connection interface by the SEM (an abbreviation of Scanning Electron Microscope) when the Sn3Ag0.5Cu solder is connected to the pad having the metallization of Au/Pd/an electroless Ni—P plating. Similarly, FIGS. 14A, 14B and 14C show the observation results of the cross section of the connection interface by the SEM when the Sn8Zn3Bi solder is connected to the pad having metallization of Au/Pd/an electroless Ni—P plating. As shown in FIG. 13, when the Sn3Ag0.5Cu solder is connected, the Sn—Cu—Ni series compound is formed at the solder/metallization interface, and the solder is reacted with the electroless Ni—P plating. However, as shown in FIG. 14, when the Sn8Zn3Bi solder is connected, the Au—Zn compound and the Pd—Zn compound are formed at the connection interface, and the solder is not reacted with the electroless Ni—P plating. For example, pp. 309–314 of “Structure of CSP connecting portion using Sn—Zn—Bi solder and joint properties, influence of Ni/Au plating” by Yamaguchi et al. Mate 2003 (2003) and pp. 57–60 of “Influence of interface structure on joint properties in CSP mounting using Sn8—Zn3—Bi solder” by Togawa et al. MES2003 (2003) show that the connection reliability is reduced when Au—Zn compound is formed at the connection interface of Sn8Zn3Bi solder and the metallization of Au/Ni plating.
Also, as the bump forming technology considered to be suitable for the thin package, pp. 272–275 of “Study of anisotropic Ni bump formation by electroless plating method and bump forming technology by dipping method” by Hatakeda MES2003 (2003) discloses the method in which a thick Ni core instead of a bump is formed by plating and solder is thinly supplied onto the Ni core by dipping, thereby forming the solder bump. According to this method, the package/substrate distance can be increased and the thermal fatigue life can be extended. Also, since the metallization does not contain Au and Pd, Au—Zn compound and Pd—Zn compound which cause the reduction of the connection interface strength are not formed even if the Sn8Zn3Bi solder is used for the connection. In this bump forming method, however, the effect of preventing the occurrence of connection failure due to the warp of the package is small, and since this forming method of the Ni core uses the plating, the time required to form the Ni core is long when extending the package/substrate distance.